The vast majority of women who quit smoking during pregnancy relapse during the postpartum period. Relapse prevention interventions are lacking. Progesterone, a female sex hormone, has been implicated in both the animal and clinical literature as being protective against addictive behaviors. This hormone increases dramatically during pregnancy (possibly offering a protective effect), then plummets soon after delivery. Therefore, the primary goal of this R21 application is to investigate the potential efficacy of exogenous progesterone (with supplemental relapse prevention counseling) on postpartum relapse in new mothers. We will also determine the feasibility of enhanced compliance monitoring and identification of collateral factors effecting outcomes - both of which will be done in preparation for a larger R01 supported rigorously controlled. To address these goals we will recruit pregnant women at gestational weeks 33-36 who have quit smoking during pregnancy and are motivated to maintain abstinence after delivery. At the time of delivery, women will be randomly assigned to receive four weeks of active (n=20) or placebo (n=20) exogenous progesterone starting on the fourth day postpartum. Participants will complete weekly clinic visits until 12 weeks postpartum to collect data on smoking status and protocol compliance, measure serum progesterone levels, and receive behavioral counseling. This study has several innovative aspects. Specifically, we will be the first to conduct a randomized clinical trial to determine whether progesterone administration postpartum can eliminate or reduce risk for postpartum smoking relapse while utilizing a daily electronic data capture protocol. This study will, therefore, advance the literature on the role of sex hormones in addictive behaviors and directly inform future relapse prevention studies for new mothers.